CO2 capture from utility flue gas is the most expensive step in an integrated carbon capture and sequestration (CCS) process. The current commercial state of the art of capture technology utilizes amine-based absorption technology. A typical, conventional process 10 using an absorption column 12 is illustrated in FIG. 1. Raw flue gas 14 enters the absorption column 12 and clean flue gas 16 exits as described below. A CO2-lean solution 18 enters into an absorption column 12 from the top and flows downward. By contacting the flue gas countercurrent, the solution absorbs most of the CO2 in the flue gas in the absorption column 12 and produces a CO2-rich solution exiting at 20. The CO2-rich solution goes through pump 22 and in line 24 goes through heat exchanger 26. After exchanging heat with the CO2-lean solution from the bottom of the stripping column 30, or stripper, the rich solution in line 28 enters the stripper 30 from the top and flows downwards. CO2 in the rich solution is stripped out by water vapor flowing upward. The heat required to strip the absorbed CO2 is entirely provided by water vapor. Line 49 provides pulls water/steam from the stripper 30 to be supplied to a reboiler 46 at the bottom of the stripper 30 with associated steam line 48. The heated water vapor from the reboiler 46 is supplied to the bottom of the stripper 30 through line 50. The CO2-lean solution in line 32 from the bottom of the stripper 30 goes through pump 34 and to the cross heat exchanger 26 through line 36. The CO2-lean solution from the stripper 30 exits heat exchanger 26 in line 38 and is then further cooled in cooling unit 40 before it enters the absorber in line 18 and the cycle repeats. Make-up solvent (amine) may be added through line 42 into the lean solution. The stripped CO2 exits the stripper 30 at the top in line 52 extending through cooler 56, having return line 58, with CO2 leaving through line 60.
A conventional absorption/stripping process is energy intensive. The heat requirement in the stripper consists of three components:Qtotal=Qsensible+Qreaction+Qstripping  (1)
Here Qreaction is the heat of reaction (also called heat of absorption), which is the same as the heat released during absorption in the absorption column; Qsensible is the sensible heat, which is the heat required to heat the CO2-rich solution from its temperature entering the stripper to the temperature of CO2-lean solution leaving the reboiler; and Qstripping is the stripping heat, that is, the heat required to generate the water vapor coming out from the top of the stripper. Each component can be calculated by the following respective equations:
                              Q          Sensible                =                                                            C                p                            ⁡                              (                                                      T                    lean                                    -                                      T                    feed                                                  )                                                    Δ              ⁢                                                          ⁢              Loading                                =                                    H              Lean                        -                          H              Rich                                                          (        2        )                                          Q          reaction                =                  Δ          ⁢                                          ⁢                      H            reaction                                              (        3        )                                          Q          stripping                =                                            (                                                P                                      H                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                    O                                                                    P                                      CO                    ⁢                                                                                  ⁢                    2                                                              )                                      Top              ⁢                                                          ⁢              of              ⁢                                                          ⁢              the              ⁢                                                          ⁢              stripper                                ×          Δ          ⁢                                          ⁢                      H                          H              ⁢                                                          ⁢              2              ⁢                                                          ⁢              O                                                          (        4        )            Here,ΔLoading is the CO2 difference per kg in solution between lean and rich;Cp is the heat capacity of the solution in kJ/kg solution;ΔHreaction and ΔHH2O are the heat of reaction and heat of vaporization of water, respectively;TA and TS are the absorption and stripping temperatures, respectively;TLean and Tfeed are the temperature of lean solution from the stripper and the temperature of the rich solution to the stripper (after cross heat exchanger);HLean and HRich are the enthalpy of the lean solution and the rich solution;PH2O and PCO2 are the partial pressures of water and CO2 respectively; andR is the gas constant.
When monoethanolamine (MEA) is used as solvent, the Qsensible, Qreaction, and Qstripping for the amine-based absorption processes are roughly 480, 800, and 270 Btu/lb CO2 respectively, with a total of around 1550 Btu/lb CO2.
There are several fundamental disadvantages to the conventional stripping processes, including:
(a) The operating pressure of the stripper is determined by vapor pressure of the CO2-lean solution in the reboiler, which in turn is determined by composition of lean solution and the reboiler temperature. In order to increase the operating pressure the temperature in the reboiler has to be raised, which is often limited by the stability of the amine solvents. The reboiler temperature in a conventional stripper is typically at 120° C. and the operating pressure is thus limited at around 28 psia.(b) Heat required for CO2 stripping is entirely provided by water vapor generated in the reboiler. Thus, water vapor is used not only as stripping gas but also as a heat carrier. Due to the dual functions of steam PH2O and PCO2 in the stripper from bottom to top are all correlated with each other.(c) Due to the low operating pressure (˜28 psia) of the stripper (thus low pressure of CO2 product), a large amount of compression work is required to compress the CO2 product to a pipeline transportation-ready pressure (˜2250 psia).
As noted above, current state-of-the-art technology for CO2 separation from post-combustion flue gas uses amine-based absorption processes. However, all amine-based absorption processes use steam as the heat carrying medium and stripping gas and thus the operating pressure of the stripper is determined by the reboiler temperature. Recent attempts to overcome the drawbacks of conventional systems include using a non-steam stripping gas into the stripping system. One would expect that the added stripping gas should be easily separated from CO2 and aqueous solution; organic vapors are therefore ideal selections. For example, in one study, an organic compound (hexane) was added into the stripping system to increase the pressure of the stripper. However, no external heat sources except heat from the reboiler were added to the stripping column and as a result, the temperature distribution within the stripper is coupled. Therefore, the energy performance of hexane stripping system was even worse than the conventional stripping system.
Others have addressed carbon dioxide recovery in a variety of applications including U.S. Patent Publication No. 2002-0081256 to Chakravarti, Shrikar, et al. discloses carbon dioxide recovery at high pressure which (A) providing a gaseous feed stream comprising carbon dioxide, wherein the pressure of said feed stream is up to 30 psia; (B) preferentially absorbing carbon dioxide from said feed stream into a liquid absorbent fluid to form a carbon dioxide enriched liquid absorbent stream; (C) in any sequence or simultaneously, pressurizing said carbon dioxide enriched liquid absorbent stream to a pressure sufficient to enable the stream to reach the top of the stripper at a pressure of 35 psia or greater, and heating the carbon dioxide enriched liquid absorbent stream to obtain a heated carbon dioxide enriched liquid absorbent stream; and (D) stripping carbon dioxide from said carbon dioxide enriched liquid absorbent stream in a stripper operating at a pressure of 35 psia or greater and recovering from said stripper a gaseous carbon dioxide product stream having a pressure of 35 psia or greater. In another aspect of this process, the stripped liquid absorbent fluid from the stripper is recycled to step (B).
U.S. Patent Publication No. 2002-0026779 to Chakravarti, Shrikar, et al. discloses a system for recovering absorbate such as carbon dioxide from an oxygen containing mixture wherein carbon dioxide is concentrated in an alkanolamine containing absorption fluid, oxygen is separated from the absorption fluid, the resulting fluid is heated, and carbon dioxide is steam stripped from the absorption fluid and recovered.
U.S. Patent Publication No. 2002-0132864 to Searle, Ronald G., discloses a method for recovering carbon dioxide from an ethylene oxide production process and using the recovered carbon dioxide as a carbon source for methanol synthesis. More specifically, carbon dioxide recovered from an ethylene oxide production process is used to produce a syngas stream. The syngas stream is then used to produce methanol.
U.S. Patent Publication No. 2004-0123737 to Filippi, Ermanno, et al. discloses a process for the separation and recovery of carbon dioxide from waste gases produced by combustible oxidation is described comprising the steps of feeding a flow of waste gas to a gas semipermeable material, separating a gaseous flow comprising high concentrated carbon dioxide from said flow of waste gas through said gas semipermeable material, and employing at least a portion of said gaseous flow comprising high concentrated carbon dioxide as feed raw material in an industrial production plant and/or stockpiling at least a portion of said gaseous flow comprising carbon dioxide.
U.S. Patent Publication No. 2004-0253159 to Hakka, Leo E., et al. discloses process for recovering CO2 from a feed gas stream comprises treating the feed gas stream with a regenerated absorbent comprising at least one tertiary amine absorbent having a pKa for the amino function of from about 6.5 to about 9 in the presence of an oxidation inhibitor to obtain a CO2 rich stream and subsequently treating the CO2rich stream to obtain the regenerated absorbent and a CO2rich product stream. The feed gas stream may also include SO2 and/or NOx.
U.S. Patent Publication No. 2006-0204425 to Kamijo, Takashi, et al. discloses an apparatus and a method for recovering CO2 are provided in which energy efficiency is improved. The apparatus for recovering CO2 includes a flow path for returning extracted, temperature risen semi-lean solution into a regeneration tower wherein at least a part of the semi-lean solution obtained by removing a partial CO2 from a rich solution infused in a regeneration tower from an upper part of the regeneration tower is extracted, raised its temperature by heat exchanging with a high-temperature waste gas in a gas duct of an industrial facility such as a boiler, and then returned into the regeneration tower.
U.S. Patent Publication No. 2006-0248890 to Iijima, Masaki, et al. discloses a carbon dioxide recovery system capable of suppressing reduction in turbine output at the time of regenerating an absorption liquid with carbon dioxide absorbed therein, a power generation system using the carbon dioxide recovery system, and a method for these systems. The carbon dioxide recovery system includes a carbon dioxide absorption tower which absorbs and removes carbon dioxide from a combustion exhaust gas of a boiler by an absorption liquid; and a regeneration tower which heats and regenerates a loaded absorption liquid with carbon dioxide absorbed therein, is characterized in that the regeneration tower is provided with plural loaded absorption liquid heating means in multiple stages, which heat the loaded absorption liquid and remove carbon dioxide in the load absorption liquid, in that a turbine driven and rotated by steam of the boiler is provided with plural lines which extract plural kinds of steam with different pressures from the turbine and which supply the plural kinds of steam to the plural loaded absorption liquid heating means as their heating sources, and in that the plural lines are connected to make the pressure of supplied steam increased from a preceding stage of the plural loaded absorption liquid heating means to a post stage of the plural loaded absorption liquid heating means.
U.S. Patent Publication No. 2007-0148068 to Burgers, Kenneth L, et al. discloses an alkanolamine absorbent solution useful in recovering carbon dioxide from feed gas streams is reclaimed by subjecting it to vaporization in two or more stages under decreasing pressures.
U.S. Patent Publication No. 2007-0148069 to Chakravarti, Shrikar, et al. discloses a system in which carbon dioxide is recovered in concentrated form from a gas feed stream also containing oxygen by absorbing carbon dioxide and oxygen into an amine solution that also contains another organic component, removing oxygen, and recovering carbon dioxide from the absorbent.
U.S. Patent Publication No. 2007-0283813 to Iijima, Masaki, et al. discloses a CO2 recovery system which includes an absorption tower and a regeneration tower. CO2 rich solution is produced in the absorption tower by absorbing CO2 from CO2-containing gas. The CO2 rich solution is conveyed to the regeneration tower where lean solution is produced from the rich solution by removing CO2. A regeneration heater heats lean solution that accumulates near a bottom portion of the regeneration tower with saturated steam thereby producing steam condensate from the saturated steam. A steam-condensate heat exchanger heats the rich solution conveyed from the absorption tower to the regeneration tower with the steam condensate. See also U.S. Patent Publication Nos. 2008-0056972; 2008-0223215; and 2009-0193970 to Iijima, Masaki, et al.
U.S. Patent Publication No. 2008-0016868 to Ochs, Thomas L., et al. discloses a method of reducing pollutants exhausted into the atmosphere from the combustion of fossil fuels. The disclosed process removes nitrogen from air for combustion, separates the solid combustion products from the gases and vapors and can capture the entire vapor/gas stream for sequestration leaving near-zero emissions.
U.S. Patent Publication No. 2008-0072752 to Kumar, Ravi discloses a vacuum pressure swing adsorption (VPSA) processes and apparatus to recover carbon dioxide having a purity of approximately 90 mole % from streams containing at least carbon dioxide and hydrogen (e.g., syngas). The feed to the carbon dioxide VPSA unit can be at super ambient pressure. The carbon dioxide VPSA unit produces three streams, a hydrogen-enriched stream, a hydrogen-depleted stream and a carbon dioxide product stream. The recovered carbon dioxide can be further upgraded, sequestered or used in applications such as enhanced oil recovery (EOR).
U.S. Patent Publication No. 2008-0159937 to Ouimet, Michel a., et al. discloses that “it has surprisingly been determined that using selected amines, a [Carbon Dioxide] capture process may be conducted using substantially reduced energy input.”
U.S. Patent Publication No. 2008-0286189 to Find, Rasmus, et al. discloses a method for recovery of high purity carbon dioxide, which is substantially free of nitrogen oxides. This reference also discloses a plant for recovery of said high purity carbon dioxide comprising an absorption column, a flash column, a stripper column, and a purification unit.
U.S. Patent Publication No. 2009-0202410 to Kawatra, Surendra K., et al. discloses a process for the capture and sequestration of carbon dioxide that would otherwise enter the atmosphere and contribute to global warming and other problems. CO2 capture is accomplished by reacting carbon dioxide in flue gas with an alkali metal carbonate, or a metal oxide, particularly containing an alkaline earth metal or iron, to form a carbonate salt. A preferred carbonate for CO2 capture is a dilute aqueous solution of additive-free (NA2 CO3). Other carbonates include (K2 CO3) or other metal ion that can produce both a carbonate and a bicarbonate salt.
U.S. Patent Publication No. 2009-0211447 to Lichtfers, Ute, et al. discloses a process for the recovery of carbon dioxide, which includes: (a) an absorption step of bringing a carbon dioxide-containing gaseous feed stream into gas-liquid contact with an absorbing fluid, whereby at least a portion of the carbon dioxide present in the gaseous stream is absorbed into the absorbing fluid to produce (i) a refined gaseous stream having a reduced carbon dioxide content and (ii) an carbon dioxide-rich absorbing fluid; and (b) a regeneration step of treating the carbon dioxide-rich absorbing fluid at a pressure of greater than 3 bar (absolute pressure) so as to liberate carbon dioxide and regenerate a carbon dioxide-lean absorbing fluid which is recycled for use in the absorption step, in which the absorbing fluid is an aqueous amine solution containing a tertiary aliphatic alkanol amine and an effective amount of a carbon dioxide absorption promoter, the tertiary aliphatic alkanol amine showing little decomposition under specified conditions of temperature and pressure under co-existence with carbon dioxide.
U.S. Patent Publication No. 2009-0235822 to Anand, Ashok K., et al discloses a CO2 system having an acid gas removal system to selectively remove CO2 from shifted syngas, the acid gas removal system including at least one stage, e.g. a flash tank, for CO2 removal from an input stream of dissolved carbon dioxide in physical solvent, the method of recovering CO2 in the acid gas removal system including: elevating a pressure of the stream of dissolved carbon dioxide in physical solvent; and elevating the temperature of the pressurized stream upstream of at least one CO2 removal stage.
U.S. Patent Publication No. 2010-0005966 to Wibberley, Louis discloses a CO2 capture method in which at an absorber station, CO2 is absorbed from a gas stream into a suitable solvent whereby to convert the solvent into a CO2-enriched medium, which is conveyed to a desorber station, typically nearer to a solar energy field than to the absorber station. Working fluid, heated in the solar energy field by insulation, is employed to effect desorption of CO2 from the CO2-enriched medium, whereby to produce separate CO2 and regenerated solvent streams. The regenerated solvent stream is recycled to the absorber station. The CO2-enriched medium and/or the regenerated solvent stream may be selectively accumulated so as to respectively optimize the timing and rate of absorption and desorption of CO2 and/or to provide storage of solar energy.
U.S. Patent Publication No. 2010-0024476 to Shah, Minish M., et al discloses a carbon dioxide recovery process in which carbon dioxide-containing gas such as flue gas and a carbon dioxide-rich stream are compressed and the combined streams are then treated to desorb moisture onto adsorbent beds and then subjected to subambient-temperature processing to produce a carbon dioxide product stream and a vent stream. The vent stream is treated to produce a carbon dioxide-depleted stream which can be used to desorb moisture from the beds, and a carbon dioxide-rich stream which is combined with the carbon dioxide-containing gas.
U.S. Patent Publication No. 2010-00037521 to Vakil, Tarun D., et al discloses a new steam reformer unit design, a hydrogen PSA unit design, a hydrogen/nitrogen enrichment unit design, and processing scheme application. The discussed result of these innovations allegedly results in re-allocating most of the total hydrogen plant CO2 emissions load to high pressure syngas stream exiting the water gas shift reactor while minimizing the CO2emissions load from the reformer furnace flue gas.
The above identified patent publications are helpful for identifying certain concepts known in the art and are incorporated herein by reference.
It would be desirable to develop a separation system and separation processes that overcome issues of the prior art systems and reduce the energy consumption of a separation process significantly.